Orthogonal frequency division multiplexing (OFDM) has been adopted as a standard for various high data rate wireless communication systems due to the spectral bandwidth efficiency, robustness to frequency selective fading channels, etc. However, implementation of the OFDM system entails several difficulties. One of the major drawbacks is the high peak-to-average power ratio, which results in intercarrier interference, high out-of-band radiation, and bit error rate performance degradation, mainly due to the nonlinearity of the high power amplifier. The peak-to-average power ratio is the peak amplitude squared divided by the RMS value squared.
One scheme considered for peak to average power reduction (PAPR) is clipping a signal in the time-domain before amplification. Clipping can reduce PAPR by 2-3 dB but can cause degradation in EVM and cause spectrum growth out-of-band. Other time-domain techniques have been proposed such as partial transmit sequence (PTS) where the signal is partitioned into sub-blocks and each sub-block is multiplied by different phase shifts to minimize the PAPR. The disadvantages of such schemes are the need to partition the transmitter signal, the complexity for searching the optimal phase shifts and the need to send side information (or blind search) at the receiver.
Frequency domain approaches have also been considered in the past such as active constellation expansion (ACE), tone injection (TI) and tone reservation (TR). In ACE and TI, the constellations are adjusted to minimize PAPR such that there is no PER loss and no need for side information at the receiver. However, there is still complexity involved in finding the best constellation points, computing the PAPR in the time-domain and iterating this process. The gains for large QAM modulation are also limited in ACE and TI. In TR, a few tones (5-15%) are reserved to help with PAPR reduction. This leads to spectral efficiency loss.